Spray devices, such as spray guns, for spraying liquids are commonly used in various industrial applications. To ensure the proper operation of a spray device, it is often necessary to monitor the flow generated by the spray device. Presently, there are applications that require rapid detection of the liquid flow generated by a spray device, and in many systems the spray device is required to guarantee that the liquid is actually sprayed when demanded by the system. For instance, in the food processing industry, spray devices are widely used to add ingredients or coating to a product. If the liquid is not sprayed on the product as programmed, significant product loss or defects can occur. Moreover, many applications use spray devices that are operated at high speed, i.e., they are turned on and off rapidly. The ability to accurately detect the flow in the high speed operation is needed to ensure the proper operation of the spray device.
In the past, different methods based on different detection principles have been used to detect liquid flows. For example, a mechanical flow device may employ pistons with magnets and reed switches, sight glass, paddle, paddle wheel, gear, or other detection mechanism. Generally, all of those flow detection devices have to be inserted in the liquid line in order to function. In other words, the flow detection is invasive because the liquid being sprayed has to come into contact with the flow sending mechanism. This can cause various issues with the reliability of the flow sensor. Most flow sensors are fluid dependent and are affected by the physical properties of the fluid, such as viscosity, specific gravity, opacity, temperature, etc., and thus have to be properly selected according to the particular liquid to be detected. Because different types of liquids may be sprayed in an application, different flow sensor technologies may be required to properly detect flows in the application, resulting in a complicated system that is difficult to set up and expensive to maintain. Moreover, insertion-type flow detectors typically are for steady-state measurements and do not have a sufficiently fast response. As a result, they are not well-suited for monitoring the flow of a spray device that is operated at a high-speed to go through rapid on-off cycling.